The present invention relates to detecting artifacts in a video signal, and more particularly to the detection of repeated and frozen frames in a video signal.
Often when video compression is involved in video transport, if the bandwidth available for transport is insufficient, a compression coder may simple re-transmit a copy of a previous frame. The defect thus created is called xe2x80x9cframe freezexe2x80x9d and is perceived by viewers as a xe2x80x9cjerkxe2x80x9d in a moving object shown in the video. Also compression coders sometimes stop encoding, i.e., they freeze. The last frame before this freezing occurs is retained and displayed by decoders, producing a still image or an image that flickers as interlaced field1 and field2 are shown in an alternating fashion.
A trivial solution for the problem of repeated or xe2x80x9cfreezexe2x80x9d frames is to subtract the corresponding fields in consecutive frames pixel by pixel. If all of the resulting pixels in the bit-map are zero, the implication is that there is a repeated frame. However such a trivial solution does not work reliably in practice because there may be random noise present in the video. Depending upon where in the video chain such noise is added, the noise pattern may not repeat from video field to video field. Therefore where the meaningful video content in consecutive frames is repeated, the difference of two fields does not produce zero since the random noise patterns do not register, i.e., they are not identical.
What is desired is a method and apparatus for the detection of repeated and frozen frames in a video signal that works reliably in a practical environment, i.e., in the presence of noise.
Accordingly the present invention provides an apparatus and method for the detection of repeated and frozen frames in a video signal by obtaining the absolute differences between pixels from corresponding fields in consecutive frames. From the absolute differences a logic bit-map is generated, a logic xe2x80x9c1xe2x80x9d being inserted into the logic bit-map for each pixel where the absolute difference exceeds a threshold that is a function of a maximum derived from the absolute differences for the field. A normalized distribution count is determined as the sum of all the logic xe2x80x9c1xe2x80x9ds in the logic bit-map. Then the logic bit-map is divided into a number of cells of equal size. The logic xe2x80x9c1xe2x80x9ds for each cell also are counted. Bounds are generated from the normalized distribution count, and the actual counts for each cell are compared with the bounds. If all of the actual counts for the field are within the bounds, a repeated frame is reported. Alternatively in a noiseless environment if all of the actual counts are less than a minimum value, a repeated frame is reported.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.